It is known that organic media, such as for example hydrocarbon streams and mixtures derived from naturally occurring oils or gasses or other fossil fuels contain small amounts of contaminants such as mercury, arsenic and lead. Especially liquid hydrocarbon mixtures such as naphtha and natural gas condensates contain both small amounts of mercury and arsenic, as well as other metals such as lead. Typical mercury and arsenic concentrations in such hydrocarbon mixtures range between about 5 and 1,000 ppb. These hydrocarbon mixtures are subjected in chemical and petrochemical plants to cracking processes, such as steam cracking or catalytic cracking, and the products of these cracking processes may be subjected to further catalytic treatments, such as hydrogenation and hydrotreatment processes.
Both mercury and arsenic can cause problems in chemical and petrochemical plants because of their toxicity and catalyst poisoning properties, especially when such catalysts contain metal components like, for example, platinum and palladium which are commonly found in hydrogenation catalysts. Apart from that, mercury can cause corrosion, for example by amalgamation, in processing equipment and especially equipment based on aluminum or its alloys. In view of the corrosive properties of mercury and the potential risk for equipment damage, it is of primary importance to remove mercury from hydrocarbon mixtures containing mercury and arsenic.
EP-A-O319615 describes a method to remove mercury in finely divided or elemental or atomic form from organic media using a solid adsorbent containing active SH-groups.
Long run tests performed by the present inventors to remove mercury from gas condensates containing both mercury and arsenic employing the solid adsorbent used in EP-A-O319615 which contains thiol groups directly attached to an aromatic ring of the polymer backbone, have shown an unsatisfactory mercury removal performance.
At a mercury level of 32 weight ppb and an arsenic level of 80 weight ppb in the feed gas condensate, the known process favored, on the long run, arsenic adsorption over mercury adsorption. The breakthrough point for mercury occurred already at thirty to forty thousand bed volumes. In typical plants such as steam crackers, at complete adsorption of mercury and arsenic, and under typical throughput rates (bed volumes of about 10 m.sup.3 ; liquid hourly space velocity of about 25 hr.sup.-1), this would mean that the adsorbent bed needs to be replaced and/or regenerated about every two months. The regeneration of the used adsorbent bed is typically done ex situ as typically the regeneration is done with concentrated HC1 which is corrosive and can attack the metal tubing. The frequency of replacement or regeneration could be decreased by using larger adsorption bed volumes, however, this is undesirable as larger beds require higher capital investment, and since distribution problems may occur with respect to the liquid to be treated. The size of the bed is also limited, as the longer the bed is, the larger the pressure drop over the bed, which pressure drop should be limited in order to avoid damaging the bed. Thus there is a need for a process for removing mercury from organic media which also contain arsenic with improved long-term mercury removal performance.